Conventionally, a catalytic combustion gas sensor is known as a sensor for detecting combustible gases such as hydrogen gas, methane gas, etc. The catalytic combustion gas sensor detects presence of a combustible gas by heating, to a predetermined temperature, a detecting element formed by causing a heat conductive layer that covers a heater coil to carry a catalyst layer, burning the combustible gas by causing the combustible gas to contact the catalyst layer, and by outputting as variation of a voltage as variation of a resistance of the heater coil according to variation of the temperature caused by the combustion heat.
FIG. 18 is a cross-sectional view showing a configuration of a conventional detecting element. FIG. 19 is a front view showing the configuration of the conventional heater coil. As shown in FIG. 18, a conventional detecting element 1 is structured such that a heater coil 12 is buried in a heat conductive layer 11 and a catalyst layer 13 is adhered to the surface of the heat conductive layer 11. As shown in FIG. 19, for the conventional heater coil 12, the portion that is buried in the heat conductive layer 11 (hereinafter, “bead portion”) is a single coil formed by winding a wire material into a coil (see, for example, Patent Document 1). Lead portions 15 extending respectively from the both ends of the bead portion 14 are not formed in a coil. In this specification, in the detecting element, a portion where the heat conductive layer and the catalyst layer covers the bead portion is referred to as “combusting portion”.
In the catalytic combustion gas sensor, a Wheatstone bridge circuit is structure with the detecting element having the above structure, a compensating element having the same structured as this detecting element and carrying an inert oxide instead of catalyst, and two resistive elements. When the resistance of the heater coil varies due to combustion heat, the variation in resistance is output as variation in voltage from the Wheatstone bridge circuit (see, for example, Patent Document 2).
As a method of manufacturing the detecting element, a method of winding a resistive wire on a core wire; coating, in this state, with an insulating agent by electro-coating; performing heat firing on the insulating agent; thereafter, exposing a non-effective portion of the resistive wire; melting the core wire; and welding the core wire to an electrode pins, is known (see, for example, Patent Document 3). According to this method, when the detecting element is manufactured, the shape of the coiled portion of the resistive wire can be prevented from collapsing.
Patent Document 1: Japanese Patent Application Laid-Open Publication No. H3-162658 (FIG. 1)
Patent Document 2: Japanese Patent Application Laid-Open Publication No. H2-59949 (FIG. 1)
Patent Document 3: Japanese Patent Application Laid-Open Publication No. S52-116289